1. Field of the Invention
This invention relates to bicycle-type drive mechanisms and, more particularly, to a multi-speed, chainless drive mechanism using a variable ratio worm gear for producing different gear ratios.
2. Description of Prior Art
Most multi-speed bicycles presently use the commonly known derailleur gear chain drive mechanism, which comprises a plurality of various sized sprockets mounted to the axle of the drive wheel, normally the rear wheel, and a shifting mechanism for moving the chain from one sprocket to another and, thus, from one gear ratio to another. The drive mechanism also comprises a spring actuated device to maintain a proper tension in the chain.
Chainless transmissions have been developed to produce drive mechanisms which are less costly, simpler and less subject to maintenance problems than the above derailleur systems. Such drive mechanisms are shown in U.S. Pat. Nos. 2,378,634, 3,863,503 and 4,005,611, which all substantially disclose a drive shaft, instead of a chain, adapted for linking gear where arrangements mounted to the pedal and drive wheel axles. The drive shaft has a gear mounted at least at one end thereof, which can longitudinally move on the shaft so as to engage with one of a number of concentric rings of gear teeth included on the gear wheel associated with that end of the shaft. The different rings provide different discrete gearing ratios.
The above patents all describe drive mechanism offering various discrete gearing ratios, whereby gear shifting is substantially rough and the effort developed by the rider instantly varies considerably when moving from one gear ratio to another.